Increasing data throughput is one of the primary challenges for new-generation communication systems based on optical fibers. High data throughput is needed to meet the needs of continually increasing demand for growing volumes of data, as well as increased telecommunication network traffic in general. This is true for both public networks and in information processing and storage centers. Due to relatively short distances, data centers usually utilize multi-mode fibers, whose throughput is mainly limited by intermodal dispersion or distortion. Networks of this type are often based on vertical cell surface emitting lasers (VCSEL) and multi-mode fibers (MMF), where the primary limitation is the effect of intermodal dispersion, i.e. the difference in group velocities, and thus in the propagation time for various modes causing spreading and even inter-symbol distortion. The negative effect of intermodal dispersion is expressed in impulse widening within a time domain, which results in limiting the signal throughput or transmission range.
Ensuring single-mode operation is also a challenge in the field of optical fiber laser structures. Single-mode lasers offer improved beam quality and, in some applications, it is necessary to use single-mode laser systems. One of possible solutions to the problems associated with MMFs is to insert a device for filtering out higher-order modes in the laser resonator to allow for the use of multi-mode fiber having a large core diameter as an active medium, in which only one mode will be effectively propagated. Ensuring single- or few-mode operation in a multi-mode fiber has a beneficial effect on beam quality, as it reduces the discrepancy in beam diffraction and outlet, and allows the beam to be focused in a smaller area to produce increased laser precision.
One of the ways to eliminate the adverse effects related to using multi-mode fibers is to use single-mode fibers (SMF) instead of multi-mode fibers. However, networks based on single-mode fibers are much more expensive to manufacture and install, and thus less eagerly used in data centers. The use of single-mode fibers as active media in lasers also has its limitations, resulting from non-linear effects occurring with small mode fields.
The literature includes numerous works on active systems for selecting and limiting the number of modes. For instance, a solution that limits the number of propagating modes and applying complex and costly external systems (optical filtering matrices) was published in an article by G. Stypniak, L. Maksymiuk and J. Siuzdak, titled “Binary phase spatial light filters for mode selective excitation of multimode fibers”, published in the Journal of Lightwave Technology 29/13 (2011). Other known methods to reduce the number of modes require precise optical systems, used to stimulate select modes only. Examples of such techniques were published, among others, in an article by L. Jeunhomme and J. P. Pocholle, titled “Selective mode excitation of graded index optical fibers”, published in Applied Optics 17/3 (1978). Due to their complex structures and high costs, the presented systems can be executed in laboratory conditions only.
Other mode filtration methods are known in which the structure of the optical fiber is modified. An example is a solution according to JP2001133647, which proposes a planar structure, where periodical changes of the lateral surface of the core cladding are used to disperse the higher-order modes leaving the core. Such a modified waveguide section can be included in the structure of a traditional optical fiber network. In addition, such an element does not require any power supply.
In turn, U.S. Pat. Pub. No 2003/169965 presents a method of filtering out higher-order modes by including in the optical fiber systems bent fibers with small-radius curvature to selectively increase higher-order mode losses. The use of small-radius curvature eliminates higher-order modes, and, as stated in the reference, transmission is more effective and faster.
Another method of modifying waveguide structure is to interfere with its chemical composition, in order to change the light propagation conditions inside the optical fiber. Such a description is included in U.S. Pat. Pub. No. 2014/286606, in which doped and non-doped regions are placed alternately, resulting in higher-order modes being scattered and attenuated.
A solution according to U.S. Pat. No. 7,194,156 presents a counterpart of an optical communication system, used to filter out higher-order modes. The structure described in this patent can be used to increase the throughput of communication systems based on multi-mode optical fibers. An element used for filtering out higher-order modes can be executed in to ways. In one of the examples, the tip of the multi-mode fiber is tapered, and the signal is led through a lens to a detector. An element constructed this way can filter out higher-order modes in a manner such that only a certain group of modes remains, where every mode reaching the detector can be treated as a separate signal. In this embodiment, signal is coupled with the use of bulk optics, which results in increasing the size of the device and additional losses.
In international patent WO2015138492 there is presented a structure for conducting quasi-multi-mode communication on multi-mode fibers. In this solution, a light signal from at least two lasers is processed by an optoelectronic transmitter optical sub-assembly (TOSA) system and coupled to a single-mode fiber in the form of a quasi-multimode signal, and then, after propagating through the transmission path, it is received by an analogue receiver optical sub-assembly (ROSA) device. The ROSA device blocks at least one higher-order mode signal.
In U.S. Pat. Pub. No. 2007/0081768 there is described an optical communication system based on multi-mode fibers, and a method to increase the throughput of this system. A part of this system is an element used to filter out higher-order modes, based on a tapering. A multi-mode optical fiber is tapered in a manner that the basic mode only is propagated in its narrowest part. A configuration for power inlet or outlet to and from a single-mode optical fiber is also presented. The system achieves the tapering using a planar tapering method.
U.S. Pat. No. 7,184,623 B2 also presents a structure that can be used to increase the throughput of telecommunication systems using multi-mode optical fibers. In this system an adiabatic coupler, possibly a tapered optical fiber, was applied to filter out higher-order modes. According to this approach, in the tapered region higher-order modes leak out of the core and are absorbed by the conventional cladding or emitted out of the cladding.
In an article by Y. Jung, G. Brambilla and D. J. Richardson, titled “Broadband single-mode operation of standard optical fibers by using a sub-wavelength optical wire filter”, published in the Optics Express 16/19 (2008), they present a method of filtering out higher-order modes by applying a fiber taper. The key is to adapt the taper geometry, particularly in the transition regions. In their proposed solution, the transition regions are adiabatic for the first-order mode and simultaneously non-adiabatic—for higher-order modes. Such a taper geometry introduces large losses for higher-order modes. In order to ensure that propagation was allowed for fundamental mode only, the taper had to be 1/100th of the diameter of the main core. The filtered modes are based on the glass-air threshold. A large taper level is required to introduce significant losses for high-order modes.
Coating the optical fiber taper region is used in evanescent field sensors. An exemplary evanescent field sensor is presented in U.S. Pat. No. 6,103,535 A, which describes the structure of a sensor based on an optical fiber taper coated with fluorophore-reacting material. Fluorescence takes place when the material coating the tapering reacts with the fluorophores in the presence of evanescent field in the tapered optical fiber. The key element of this reference is the material used to coat the tapering. In contact with a specific substance, this material undergoes a chemical reaction, which causes fluorescence. This type of sensor can be used with multi-mode optical fibers, which guarantee higher sensor sensitivity.
Coating an optical fiber tapered region can also be used in the structure of impulse mode-locked fiber lasers. In their publication titled “A practical topological insulator saturable absorber for mode-locked fiber laser” and published in Nature 5/8690 (2015), P. Yan, R. Lin, Sh. Ruan, A Liu, H. Chen, Y. Zheng, S. Chen, C. Guo and J. Hu presented a tapered, single-mode optical fiber SMF-28 with an saturable absorbent, which is characterized by its transparency at certain wavelengths while the signal intensity is high enough.
U.S. Pat. No. 8,384,991 discloses an invention consisting of an saturable absorbent which could be applied, among others, in the structures of impulse fiber lasers. The subject of that invention is a special substance, a mixture of carbon nanotubes and polymer composites. A fragment of the tapered optical fiber coated with this substance, is used in the structure of the impulse fiber laser.
U.S. Pat. No. 6,301,408 discloses an invention consisting of a fiber Bragg grating inscribed in an optical fiber taper region, where the taper region is coated with a special polymer. By adapting a suitable refraction index in the taper's polymer coating, it is possible to control the Bragg wavelength. According to the disclosure, the proposed structure can be used to build an add/drop multiplexer, but the properties of the fundamental mode are changed.
Therefore, a need exists to overcome the problems with the prior art as discussed above.